Problem: In history class, the girl to boy ratio is $6$ to $4$. If there are $18$ girls, how many boys are there?
A ratio of $6$ girls to $4$ boys means that there is a group of $6$ girls for every group of $4$ boys. If there are $18$ girls, then there are $3$ groups of $6$ girls each. Because we know the ratio is $6:4$ , there must be $3$ groups of $4$ boys each. There is a total of $12$ boys in history class.